The present invention relates to an image pickup apparatus and a focus control method for performing a focus control using an image (a video) generated by using an image pickup device.
A video camera or a digital still camera that includes an image pickup device such as a CCD sensor and a CMOS sensor commonly generates a focus evaluation signal (an AF evaluated value) showing a contrast state of a video from a video signal to perform a focus control so as to maximize the AF evaluated value. That is a so-called contrast AF.
An image pickup apparatus using an image pickup device of a so-called rolling shutter method such as a CMOS sensor can read signals at a speed higher than that of a conventional apparatus (in a cycle shorter than that of a convention apparatus) and generate a unit image (a frame image, or a field image constituting the frame image) based on the signals. Therefore, the focus evaluation signal can also be generated in a cycle shorter than that of the conventional apparatus and the accuracy and the response of the focus control can be improved.
However, when an object illuminated by a light source that causes flickers such as a fluorescent light is imaged by using such an image pickup device that can read at high speed, a luminance variation in each of unit images by the flickers affects the focus evaluation signal.
Japanese Patent Laid-Open No. 2001-186407 discloses a technology for obtaining a video from which the luminance variation by the flickers of the fluorescent light has been removed. This performs a gain adjustment (an amplification) in accordance with the luminance for the unit image generated based on the signals read from the image pickup device. Thus, a video with a small luminance variation can be obtained. Japanese Patent Laid-Open No. 2001-186407, however, does not consider the focus control (a contrast AF).
If the focus control signal is generated from the video signal from which the luminance variation has been reduced by the technology disclosed in Japanese Patent Laid-Open No. 2001-186407, the effect of the luminance variation for the focus control signal might be able to be reduced.
However, as described above, the gain adjustment has been performed for the unit image that is a base for generating the focus control signal. The gain adjusted image commonly declines in an S/N ratio. The focus evaluation signal generated based on the unit image with a low S/N ratio frequently has errors. If such a focus evaluation signal is used, a fine contrast AF cannot be performed.